System and method for determining the start time of a pressure pulse from a downhole explosive device

ABSTRACT

A system and method for use in determining the start time of a pressure pulse created by a downhole explosive device detonated by a firing signal includes a downhole tool with a pressure pulse detector that detects the pressure pulse and generates an input signal in response thereto, and a circuit path configured to output a signaling pulse distinguishable from the firing signal. A surface unit receives and discriminates the signaling pulse from the firing signal, and outputs a start time signal in response to receiving the signaling pulse. Alternatively, the downhole tool includes a processor that measures and stores the time that the pressure pulse detector generates the input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Provisional Patent Application No. 61/977,409 filed on Apr. 9, 2014 entitled “Downhole Perforation Timing System”, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system and method for determining the start time of a downhole pressure pulse, such as created by a downhole explosive device.

BACKGROUND OF THE INVENTION

The conventional method used to determine the location of micro-seismic events (i.e., those events resulting from induced seismicity related to hydraulic fracturing) relies upon velocity models derived from dipole sonic logs. An inversion process is used to minimize the misfit error between actual and theoretical event arrival times using a multiplicity of receivers. However, sonic tools use acoustic energy in the 10,000 to 30,000 Hz range to measure formation velocities, a range which greatly exceeds the typical frequency content of micro-seismic events. Since seismic energy in the earth is dispersive (i.e., velocity varies with frequency), error is introduced into the velocity model.

The seismic energy from multiple perforation shots from a perforating gun can be used to calibrate and improve the velocity model derived from sonic logs. Receivers are positioned in a nearby borehole, on the surface, or both, to detect pressure pulses created by the perforation shots. Every ray path between a perforation shot location and a receiver contributes to a more complete mapping of the three-dimensional velocity structure. If both the ray path distance and ray path travel time are known, then the pressure pulse velocity can be determined for each ray path. Information regarding velocity anisotropy enhances the understanding of the petro-physical properties of the rock, leading to a better correlation with surface seismic data, and a further reduction in event location error.

In order to accurately determine the ray path travel time, it is necessary to accurately determine the detonation time of the explosive device used in the perforating gun. Unfortunately, the firing signal used to detonate the explosive device is not a reliable indicator of the actual detonation time because of variable and unpredictable delays between the generation of the firing signal and the detonation of the explosive device. For example, blasting caps typically have moisture sensors. If downhole moisture invades the blasting cap, this can delay detonation by wetting electrical conductors between the firing system and the explosive device itself. The firing system may have a safety lockout which introduces a time delay between generation of the firing signal and its transmission to the explosive device. Different firing systems are associated with different delays between sending the firing signal and actual detonation. Further, the electrical firing signal typically interferes with any up-hole transmitted signaling pulses. It has thus not been possible to reliably and accurately determine when the explosive was actually detonated downhole.

SUMMARY OF THE INVENTION

The present invention relates to a system and method for determining the start time of a downhole pressure pulse created by the firing of a downhole explosive device.

In one aspect, the present invention comprises a downhole tool for creating a signaling pulse in response to a pressure pulse created by a downhole explosive device detonated by a firing signal. The downhole tool comprises a pressure pulse detector for detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse, and a circuit path operatively connected to the pressure pulse detector, and configured to output a signaling pulse distinguishable from the firing signal, in response to the input signal.

In one embodiment, the downhole tool is configured as a sub that can be lowered downhole by a wireline.

In one embodiment, the downhole tool is powered solely by a power source located at the surface and used to create the firing signal.

In one embodiment, the downhole tool circuit path is further configured to amplify the input signal.

In one embodiment, the downhole tool circuit path is further configured to discriminate the input signal from the firing signal by comparing a parameter of the input signal to a predetermined parameter associated with the firing signal, and to output the signaling pulse only if the input signal is discriminated from the predetermined parameter.

In one embodiment, the downhole tool circuit path is configured to output the signaling pulse with a greater voltage, a greater current, or both a greater voltage and a greater current than the firing signal.

In another aspect, the present invention comprises a system for creating a start time signal in response to a pressure pulse created by a downhole explosive device detonated by a firing signal. The system comprises a downhole tool, as described above, and a surface unit. The surface unit comprises a circuit path operatively connected to the downhole tool circuit path, and configured to discriminate the signaling pulse from the firing signal, and to output the start time signal in response to receiving the signaling pulse.

In one embodiment, the surface unit circuit path is configured to discriminate the signaling pulse from the firing signal by comparing a parameter of the signaling pulse to a predetermined parameter associated with the firing signal.

In one embodiment, the surface unit circuit path is configured to analogically attenuate electrical noise associated with the firing signal that is transmitted with the signaling pulse.

In one embodiment, the surface unit circuit path is configured to digitally extract the firing signal from the signaling pulse to output the start time signal.

In one embodiment, the downhole tool circuit path is operatively connected to the surface unit circuit path by a transmission path that also transmits the firing signal to the downhole explosive device.

In another aspect, the present invention comprises a method for creating a start time signal in response to a pressure pulse created by a downhole explosive device detonated by a firing signal, the method comprising the steps of:

(a) using a downhole tool for:

-   -   (i) detecting the pressure pulse and generating an input signal         in response to detecting the pressure pulse;     -   (ii) in response to detecting the pressure pulse, generating a         signaling pulse distinguishable from the firing signal;

(b) using a surface unit for:

-   -   (i) receiving the signaling pulse;     -   (ii) discriminating between the signaling pulse and the firing         signal; and     -   (iii) in response to receiving the signaling pulse, outputting         the start time signal.

In one embodiment, the downhole explosive device comprises a perforating gun or an electric blasting cap.

In one embodiment, the downhole tool is powered solely by a power source located at the surface and used to create the firing signal.

In one embodiment, the downhole tool is further used for amplifying the input signal.

In one embodiment, the downhole tool is further used for discriminating the input signal from the firing signal by comparing a parameter of the input signal to a predetermined parameter associated with the firing signal, and wherein generating the signaling pulse is conditional on the input signal being discriminated from the firing signal.

In one embodiment, the signaling pulse is distinguishable from the firing pulse by a greater voltage, a greater current, or both a greater voltage and a greater current than does the firing signal.

In one embodiment, the surface unit discriminates the signaling pulse from the firing signal by comparing a parameter of the signaling pulse to a predetermined parameter associated with the firing signal.

In one embodiment, the method further comprises, before receiving the signaling pulse at the surface unit, the step of analogically attenuating electrical noise associated with the firing signal that is transmitted with the signaling pulse to the surface unit.

In one embodiment, the method further comprises, before receiving the signaling pulse at the surface unit, the step of digitally extracting the firing signal from the signaling pulse to output the start time signal.

In one embodiment, the surface unit receives the signaling pulse in a transmission path that also transmits the firing signal to the downhole explosive device.

In another aspect, the present invention provides a downhole tool for determining the time of a pressure pulse created by a downhole explosive device detonated by a firing signal. The downhole tool comprises a pressure pulse detector for detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse, and a processor operatively connected to the pressure pulse detector by a circuit path. The processor comprises an internal clock and a memory component storing a set of instructions executable by the processor to determine a time at which the processor receives the input signal, and store the time in the memory component.

In one embodiment, the downhole tool is configured by a sub that can be lowered downhole by a wireline.

In one embodiment, the downhole tool is powered solely by a power source located at the surface and used to create the firing signal.

In one embodiment, the downhole tool circuit path is further configured to amplify the input signal.

In one embodiment, the downhole tool circuit path is configured to discriminate the input signal from the firing signal by comparing a parameter of the input signal to a predetermined parameter associated with the firing signal, and to transmit the input signal to the processor only if the input signal is discriminated from the firing signal.

In one embodiment, the set of instructions is executable by the processor to further store the input signal in the memory component in association with the time.

Additional aspects and advantages of the present invention will be apparent in view of the description, which follows. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of an exemplary embodiment with reference to the accompanying simplified, diagrammatic, not-to-scale drawings.

FIG. 1 is a schematic block diagram showing use of one embodiment of a system of the present invention in a field installation.

FIG. 2 is a schematic block diagram of one embodiment of a downhole tool of the system of FIG. 1.

FIG. 3 is a schematic diagram of one embodiment of a circuit path of a downhole tool of the present invention.

FIG. 4 is a schematic block diagram of one embodiment of a surface unit of the system of FIG. 1.

FIG. 5 is a schematic diagram of one embodiment of a circuit path of a surface unit of the present invention.

FIG. 6 is a schematic diagram of an alternative embodiment of a circuit path of a surface unit of the present invention.

FIG. 7 is a schematic depiction of one embodiment of a memory-based downhole tool of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates generally to systems and methods for determining a start time of a downhole pressure pulse created by the firing of a downhole explosive. When describing the present invention, all terms not defined herein have their common art-recognized meanings. To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention. The following description is intended to cover all alternatives, modifications and equivalents that are included in the spirit and scope of the invention, as defined in the appended claims.

As used herein, the term “explosive device” refers to any type of device capable of creating a “pressure pulse” in a surrounding medium upon the explosive device's activation or detonation. Suitable explosive devices include, but are not limited to, a perforating gun, an electric blasting cap, and the like. As used herein, the term “pressure pulse” refers to a pressure variation that propagates in a gas, liquid or solid medium, and includes sound, vibration, ultrasound, and infrasound waves and pulses. Sound propagates primarily as a pressure wave or pulse.

One embodiment of the system (10) of the present invention as shown in FIG. 1 comprises a downhole tool (12) and a surface unit (14), with such components being operatively connected. In one embodiment, the surface unit (14) is also operatively connected to a firing device (16) which generates a firing signal to detonate a downhole explosive device (18) positioned within a wellbore (20). As used herein, the term “operatively connected” in describing the relationship between components means a connection for conveying signals between components, such as, for example, the electric wireline (22), the cable (24), or a wireless transmitter and receiver. In one embodiment, the electrical wireline (22) is the same transmission path that is used by the firing device (16) to transmit the firing signal to the explosive device (18).

In general, the downhole tool (12) detects a pressure pulse (36) created by the detonation of the downhole explosive device (18), using for example a pressure transducer, and transmits a signaling pulse (44) up-hole through the electric wireline (22) to the surface unit (14), which can be used to provide accurate indicator of the time at which the firing of the explosive device (18) occurred downhole (t=0). The surface unit (14) processes the signaling pulse (44) to provide a start time signal (54). The system (10) may be used in conjunction with a remotely located receiver (not shown) which generates a signal in response to the pressure pulse. A computer (not shown) equipped with a timing system is operatively connected to the surface unit (14) to determine the time elapsed between the start time signal (54) and the signal subsequently received from the receiver. The elapsed time represents the travel time of the pressure pulse from the downhole tool (12) to the remote receiver. This travel time can be used to determine the velocity of the pressure pulse if the distance between the downhole tool (12) and the remote receiver is known.

As shown in one embodiment in FIG. 2, the downhole tool (12) comprises a pressure pulse detector (26) operatively connected to a circuit path comprising an amplifier (28), a discriminator and logic unit (30), a signal generator (32), and a power supply (34).

The pressure pulse detector (26) detects an acoustic pressure pulse (36) created by the firing of the explosive device (18), and generates an input signal (38) in response to detecting the pressure pulse (36). The pressure pulse detector (26) may be any suitable device known in the art, such as a pressure transducer, which without limitation, may include piezoresistive, capacitive, electromagnetic, or optical devices.

The amplifier (28) increases the amplitude of the input signal (38) to yield a relatively larger output signal (40). In one embodiment, the amplifier (28) modulates the output signal (40) to match the input signal (38) shape, but with a relatively larger amplitude. The amplifier (28) transmits the amplified output signal (40) to the discriminator and logic unit (30).

In one embodiment, the pressure pulse detector (26) produces an analog signal (38). The discriminator of the discriminator and logic unit (30) converts the analog input signal (38), after amplified by amplifier (28), into a standardized output pulse (42) whenever the amplified input signal (38) amplitude exceeds a predetermined threshold voltage. The logic unit of the discriminator and logic unit (30) performs the logical operations (for example, AND, NAND, OR, NOR and NOT). The input signal (38) and output pulse (42) amplitude corresponds to two possible states: “0” (or “TRUE”) or “1” (“FALSE”). In one embodiment, the logic unit signals are joined so that the output pulse (42) is “1” or “TRUE” only when the input signal (38) corresponds to a predetermined pattern. In one embodiment as shown in FIG. 3, the discriminator and logic cunit (30) comprises one or more comparators to compare the voltage of the input signal (38) to the predetermined threshold voltage and outputs a “1” or “TRUE” output pulse (42) if the input signal (38) exceeds the predetermined threshold voltage. One comparator may be configured for a negative signal and the other comparator may be configured for a positive signal to accommodate input signals (38) of either polarity. The output pulse (42) from the discriminator and logic unit (30) is then sent to the signal generator (32).

The signal generator (32) receives the output pulse (42) from the discriminator and logic unit (30), and processes the output pulse (42) into a signaling pulse (44) distinguishable from the firing signal. In one embodiment, the signal generator (32) amplifies the energy of the signaling pulse (44) so that it is greater than the energy of the firing signal in order to be detectable by the surface unit (14). The signaling pulse (44) may have a greater voltage, or a larger current, or both, than the firing signal. In other embodiments, the signaling pulse (44) may have other distinguishable parameters in comparison with the firing signal. In one embodiment as shown in FIG. 3, the signal generator (32) may comprise one or more integrated circuits that amplify the output pulse (42) to produce the signaling pulse (42). The signal generator (32) transmits the high energy signaling pulse (44) to the surface unit (14).

The power supply (34) provides operating power (46) to the amplifier (28), the discriminator and logic unit (30), and the signal generator (32). In one embodiment, as a safety feature, the downhole tool (12) lacks any intrinsic power source which might accidentally detonate the explosive device (18). The power supply (34) uses power (46) from the same source which operates the firing device (16) to initiate the detonation of the explosive device (18). In one embodiment, the operating power (46) is about 12 V DC. In one embodiment, the power supply (34) is “fast settling” in the sense that it quickly stabilizes so that the amplifier (28) and the discriminator (30) are ready to detect an input signal (38) generated by the pressure pulse detector (26).

By transmitting the signaling pulse (44) to the surface unit (14), the downhole tool (12) thus provides the surface unit (14) with a relatively accurate indicator of the time at which the firing of the explosive device (18) occurred. The wireline delay—i.e., the time required by the signaling pulse (44) to travel through the electric wireline (22) to the surface unit—may be the major source of inaccuracy, but is practically very brief. In one embodiment, the wireline delay may vary by ±10 μs because of transmission velocity of the signaling pulse (44) through the electric wireline (22). This wireline delay depends upon the length of the wireline or electrical connection (22). The wireline delay is typically estimated to be about 40 μs for a wireline having a length of about 5000 m. As the wireline delay is a measurable constant delay, it can be predicted or measured, and accounted for, if greater accuracy is required in determining the firing time of the downhole explosive device (18).

In one embodiment, the downhole tool (12) may be configured as a sub which can be lowered downhole by the electric wireline (22) to be positioned proximate to the explosive device (18). The sub is preferably rated to withstand elevated temperature and pressure, in one example, a minimum of about 100° C. and about 135 MPa.

As shown in FIG. 4, in one embodiment, the surface unit (14) comprises a circuit path comprising an analog signal filter (48), a digital filter (49), discriminator (50), and a power source (60). The circuit path of the surface unit (14) receives the high energy signaling pulse (44) from the circuit path of the downhole tool (12) through an operative connection, such as the electric wireline (22).

It will be appreciated that in practical implementation, the time that elapses between the transmission of the firing signal and transmission of the signaling pulse (44) is very brief. If the firing signal and signaling pulse (44) are transmitted on the same electric wireline (22) or electric wirelines in proximity to each other, the signaling pulse may be contaminated with the firing signal, and possibly interference signal from other devices. Thus, in one embodiment, the analog filter (48) and digital filter (49) conduct signal separation and signal restoration on the signaling pulse (44) received from the downhole tool (12). Signal separation is needed when a signal has been contaminated with interference, noise, or other signals such as interference signals associated with the firing signal or firing device. Signal restoration is used when a signal has been distorted in some manner. The analog signal filter (48) processes the signaling pulse (44) received from the downhole tool (12) by attenuating any electrical firing noise. In embodiments as shown in FIGS. 5 and 6, the analog filter (48) comprises one or more resistor and capacitor elements. The analog signal filter (48) transmits the filtered signal (52) to the digital filter (49) for further processing.

The digital filter (49) enhances the filtered signal (52) to output a start time signal (54) to a computer system (not shown). In embodiments, as shown in FIGS. 5 and 6, the digital filter (49) comprises one or more integrated circuits. The integrated circuits are programmed or programmable with algorithms that account for known firing signals and interference signal generated by different firing devices (16). In this manner, the digital filter (49) may be customized and adapted for use with a variety of different firing devices (16). The algorithms subtract the known firing signal and interference signal generated by a selected firing device from the filtered signal (52), thus extracting only the signaling pulse (44) for use as the start time signal (54).

The discriminator (50) tracks the firing voltages and sets the detection levels from the filtered signal (52) so as to distinguish it from the firing signal or other interference signals. In embodiments, as shown in the FIGS. 5 and 6, the discriminator comprises one or more comparators to compare the voltage of the filtered signal (52) to a pre-determined threshold voltage associated with the firing signal for a standard blasting cap. If the voltage of the filtered signal (52) exceeds the pre-determined threshold voltage, the comparator outputs a “1” or “TRUE” start time signal (54). One comparator may be configured for a negative signal and the other comparator may be configured for a positive signal to accommodate filtered signals (38) of either polarity. In one embodiment as shown in FIG. 6, the filtered signal (52) may by-pass the discriminator (50), and proceed directly to the digital filter (49) for processing, as described above. In such an embodiment, the digital filter (49) effectively discriminates the filtered signal (52) from the firing signal or other interference signals.

The power source (60) provides operating power to the components of the surface unit (14).

In one embodiment, at least one noise filter (56) is included on each of the lines (22, 24) between the surface unit (14) and the firing device (16), respectively, to filter such unwanted components such as background noise or interfering signals. Optionally, a custom firing power supply (58) may be used to activate or detonate the explosive device (18), which may enhance the signal to noise ratio.

A display (not shown) which is either operatively connected to or integral with the surface unit (14) may display indication signals (for example, system status, errors, alarms, output messages, instructions, audible buzzers, indicator lights) to perform a test of the system (10) to ensure proper connection of all components before operation, and to inform a user whether firing of the downhole explosive device (18) has been detected. For example, a lengthy continuous beep or illumination might indicate that the explosive device (18) has detonated successfully, and a short fast beep or illumination might indicate that the explosive device (18) has misfired or a malfunction has occurred in the system (10).

The surface unit (14) may include an operational switch so that a user can specify the particular type of explosive device (18) from which the start time signal pulse (54) will originate. Types of explosive devices (18) to which the surface unit (14) may be responsive include, but are not limited to, non-radio frequency type detonators, PX-1™ detonators (Teledyne RISI, Inc., Tracy, Calif.), and DynaEnergetics™ detonators (DynaEnergetics GmbH & Co. KG, Troisdorf, Germany). As discussed above, the digital filter (49) may be configured with algorithms that account for known firing signals and interference signal generated by firing devices (16) used with different types of explosive devices (18).

A remote unit (not shown) may be included in the system (10) in order to transmit the start time signal pulse output (54) to a separate remote unit (not shown) to be read at a different location or site. Display means may be integral with the remote unit to display indication signals (for example, audible buzzers, indicator lights) to confirm firing of the downhole explosive device (18) in order that the user can then transmit the start time signal pulse (54) to the remote unit. Transmitting acoustic data remotely conveniently enables another user to obtain the data without having to read the display of the surface unit (14) in person, or risk injury by being present in a detonation area.

In use and operation, a computer which comprises a timing clock system may be operatively connected to both the surface unit (14) and a remote receiver comprising conventional acoustic measuring equipment. The computer may be a separate physical component, physically integrated with the surface unit (14) or the remote receiver, or a combination of the foregoing. The remote receiver generates a signal upon being actuated by the pressure pulse. Since the distance between the remote receiver and the explosive device is greater than the distance between the remote receiver and the downhole tool (12), the computer receives the signal generated by the remote receiver after receiving the start time signal from the surface unit (14). The computer uses start time signal (54) transmitted from the surface unit (14) to accurately mark the time of detonation of the explosive device (18), and uses the signal transmitted from the remote receiver to accurately mark the time that the pressure pulse reached the remote receiver. By knowing these two times, it is possible to accurately determine an elapsed time for the acoustic pulse to travel to the receiver based on the difference between these two times, and accounting for wireline delay as necessary. The velocity of the pressure pulse between the explosive device (18) and the remote receiver may be determined if the distance between them is known.

In an alternative embodiment as shown in FIG. 7, a memory-based downhole tool (120) is provided. In this case, the memory downhole tool (120) is configured similarly to downhole tool (12) shown in FIG. 2 in that it comprises a pressure pulse detector (126), an amplifier (128), and a discriminator and logic unit (130) that operate in a similar manner to the corresponding components of the downhole tool (12) described above. However, in this embodiment, the memory-based downhole tool (120) need not be operatively connected to a surface unit (14). Rather than transmitting the output pulse (42), to a signal generator (32) and uphole to the surface unit (14), the output pulse (42) is received by a processor (132) having an internal clock and a memory component, which forms part of the memory-based downhole tool (20). The internal clock may include temperature compensation, as is well known in the art. In response to the output pulse (42), the processor (132) determines the timing of the output pulse and stores the output pulse (36) into the memory component with an associated time tag, in a time data file. In this embodiment, a battery power supply (150) may be used to power the amplifier (128), the discriminator and logic unit (130) and the processor (132). In one embodiment, there is no electrical connection between the electric wireline (22) that is used to power the explosive device (18) and the battery-powered circuit path of the memory-based downhole tool (120) to reduce the possibility of the battery power supply (130) accidentally activating or detonating the explosive device.

Before use downhole, the processor (132) of the memory-based downhole tool (120) is operatively connected to a computer (not shown) by any conventional means (160), such as USB, serial port or wireless means such as Bluetooth or WiFi. The computer includes or is connected to a standalone timing clock system, which the computer synchronizes to the internal clock of the memory downhole tool (120). In this manner, the internal clock of the memory downhole tool (120) may be synchronized with a timing clock system that is used to determine when an output signal is received from a remote receiver.

The memory-based downhole tool (120) may then be deployed and used downhole to detect pressure pulses.

The surface timing clock system may also be connected to a receiving device (conventional seismic device) and receives time entries corresponding to pressure pulses received by the remote receiver. When the memory downhole tool (120) is brought back to the surface, it is re-connected to the computer and resynchronized with the surface timing clock system. The time difference drift from the memory downhole tool (120) can then be determined and corrected.

By knowing the time tagged to the output pulse (42) by the processor (142) and the time that the remote receiver generated the output pulse, it is possible to accurately determine an elapsed time for the acoustic pulse to travel to the receiver based on the difference between these two times. The velocity of the pressure pulse may be determined if the distance between the explosive device (18) and the remote receiver is known.

The flowchart and block diagrams and schematics in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

What is claimed is:
 1. A system for creating a start time signal in response to a pressure pulse created by a downhole explosive device detonated by a firing signal, the system comprising: (a) a downhole tool comprising: (i) a pressure pulse detector for detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse; and (ii) a circuit path operatively connected to the pressure pulse detector, and configured to output a signaling pulse having a signal parameter distinguishable from the signal parameter of the firing signal, in response to the input signal; and (b) a surface unit comprising a circuit path operatively connected to the downhole tool circuit path, and configured to discriminate the signaling pulse from the firing signal, and to output the start time signal in response to receiving the signaling pulse.
 2. The system of claim 1, wherein the downhole tool is powered solely by a power source located at the surface.
 3. The system of claim 1, wherein the downhole tool circuit path is further configured to amplify the input signal.
 4. The system of claim 1 wherein the downhole tool circuit path is further configured to discriminate the input signal from the firing signal by comparing a parameter of the input signal to a predetermined parameter associated with the firing signal, and to output the signaling pulse only if the input signal is discriminated from the predetermined parameter associated with the firing signal.
 5. The system of claim 1 wherein the downhole tool circuit path is configured to output the signaling pulse with a greater voltage, a greater current, or both a greater voltage and a greater current than the firing signal.
 6. The system of claim 1 wherein the surface unit circuit path is configured to discriminate the signaling pulse from the firing signal by comparing a parameter of the signaling pulse to a predetermined parameter associated with the firing signal.
 7. The system of claim 1 wherein the surface unit circuit path is configured to analogically attenuate electrical noise associated with the firing signal that is transmitted with the signaling pulse and/or is configured to digitally extract the firing signal from the signaling pulse to output the start time signal.
 8. The system of claim 1 wherein the downhole tool circuit path is operatively connected to the surface unit circuit path by a transmission path that also transmits the firing signal to the downhole explosive device.
 9. A method for creating a start time signal in response to a pressure pulse created by a downhole explosive device detonated by a firing signal, the method comprising the steps of: (a) using a downhole tool for: (i) detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse; (ii) in response to detecting the pressure pulse, generating a signaling pulse distinguishable from the firing signal; (b) using a surface unit for: (i) receiving the signaling pulse; (ii) discriminating between the signaling pulse and the firing signal; and (iii) in response to receiving the signaling pulse, outputting the start time signal.
 10. The method of claim 9, wherein the downhole tool is powered solely by a power source located at the surface.
 11. The method of claim 9 wherein the downhole tool is further used for amplifying the input signal.
 12. The method of claim 9 wherein the downhole tool is further used for discriminating the input signal from the firing signal by comparing a parameter of the input signal to a predetermined parameter associated with the firing signal, and wherein generating the signaling pulse is conditional on the input signal being discriminated from the firing signal.
 13. The method of claim 9 wherein the signaling pulse is distinguishable from the firing pulse by a greater voltage, a greater current, or both a greater voltage and a greater current than does the firing signal.
 14. The method of claim 9 wherein the surface unit discriminates the signaling pulse from the firing signal by comparing a parameter of the signaling pulse to a predetermined parameter associated with the firing signal.
 15. The method of claim 9 further comprising, before receiving the signaling pulse at the surface unit, the step of analogically attenuating electrical noise associated with the firing signal that is transmitted with the signaling pulse to the surface unit and/or the step of digitally extracting the firing signal from the signaling pulse to output the start time signal.
 16. The method of claim 9 wherein the surface unit receives the signaling pulse in a transmission path that also transmits the firing signal to the downhole explosive device.
 17. A downhole tool for creating a signaling pulse in response to a pressure pulse created by a downhole explosive device detonated by a firing signal, the downhole tool comprising: (a) a pressure pulse detector for detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse; and (b) a circuit path operatively connected to the pressure pulse detector, and configured to output a output the signaling pulse distinguishable from the firing signal, in response to the input signal.
 18. The downhole tool of claim 17 wherein the downhole tool is powered solely by a power source located at the surface and used to create the firing signal.
 19. A downhole tool for determining the time of a pressure pulse created by a downhole explosive device detonated by a firing signal, the downhole tool comprising: (a) a pressure pulse detector for detecting the pressure pulse and generating an input signal in response to detecting the pressure pulse; and (b) a processor operatively connected to the pressure pulse detector by a circuit path, the processor comprising an internal clock and a memory component storing a set of instructions executable by the processor to determine a time at which the processor receives the input signal, and store the time in the memory component.
 20. The downhole tool of claim 19 wherein the set of instructions is executable by the processor to further store the input signal in the memory component in association with the time. 